The present invention relates to the improvement of a controller for power converter which determines and controls the firing phase of the power converter by the current control circuit which is supplied with the current difference between a current target value to be supplied to a load and a current actual value.
Power converters for supplying to a load the power which can be changed by the firing phase control have been widely used in various fields as is well known. For example, a power converter formed by thyristors in Greatz connection is used for driving a DC motor.
When an inductive load such as a motor is drived by a power converter, the current flowing in the power converter takes a continuous or an intermittent conduction state depending on the load condition. As is generally known, the currentless period of time becomes long in the intermittent region, thus greatly changing the gain of the current control circuit which is supplied with the current difference between a target value of current to be applied to a load and an actual value of current frowing therethrough so as to determine the firing phase of the power converter in accordance with the current difference. The current control circuit has its gain greatly reduced in the current intermittent region. Therefore, the response characteristic of the current control circuit must be determined in response to the current intermittent region, and thus the response characteristic thereof in the current continuous region will deteriorate and be unstable.
To solve such a problem, there has hitherto been proposed a nonlinear compensation method by which the nonlinearity of the gain characteristic thereof can be compensated for upon the intermittent current flow as will be described below.
A nonlinear compensating circuit is provided which produces a correction angle for the firing phase (delayed control angle) when an actual value of current to a load is reduced to less than a value at the boundary between the continuous and intermittent current regions (hereinafter, referred to as the current boundary value). This correction angle and a firing angle instruction from the current control circuit are added to be a firing control angle for the power converter, thereby effecting the nonlinear compensation.
However, this nonlinear compensation method has the following defects because the nonlinear compensation is performed in the current intermittent region at all times.
In the nonlinear compensation method, the correction angle is reduced with increase of the actual value of current in the current intermittent region. This means that the increase of the actual value of current reduces the firing control angle, thus increasing the actual value of current. Thus, the nonlinear compensation loop of the nonlinear compensation circuit serves as a positive feedback loop with respect to the current control circuit, thereby impairing the stability of the condition at which the current target value and the current actual value are coincident with each other in the current intermittent region, that is, the stability of the steady-state condition. If the gain of the positive feedback loop of the nonlinear compensation circuit is increased to be larger than that of the negative feedback loop of the current control circuit particularly in order to improve the responsibility of compensation for non-linearity, the effect of the positive feedback loop is dominant thereby causing a hunting in the steady state condition.
On the other hand, if the gain of the negative feedback loop is increased larger than that of the positive feedback loop in order to ensure the stability of the steady state condition in the current intermittent region, the positive feedback loop can not provide an effective nonlinear compensation.
Thus, the nonlinear compensation method is able to improve the response in the current intermittent region, but makes the steady-state condition unstable.